Ever since the tsunami hit exactly three years ago, Fukushima Daiichi has been an ongoing disaster scene -- glowing infernal heat, radioactive water leaks, and in at least one section, the dose rate is 54 millisieverts per hour, a year’s allowable dose. Removing every trace of the power station and its three melted nuclear cores is going to take some advanced robotics, which Japan’s biggest tech companies are rushing to invent and build. Eliza Strickland reports for IEEE Spectrum
This giant demolition project will likely take 40 years to complete and cost $15 billion. And while much of the tech already exists in some form, it must be adapted to fit these never-before-seen circumstances. “It’s like in the 1960s, when we wanted to put a man on the moon,” Lake Barrett, who consults for the cleanup, tells IEEE Spectrum
. “We had rocketry, we had physics, but we had never put all the technologies together.”
During the first three years of the cleanup, TEPCO has been surveying the site to create maps of radiation levels. The next step is removing radioactive debris and scrubbing radioactive materials off walls and floors. Spent fuel must be removed from the pools in the reactor buildings; leaks must be plugged. Only then will workers be able to flood the containment structures so that the melted globs of nuclear fuel can safely be broken up, transferred to casks, and carted away.
Here’s a detailed look at the steps for this decommissioning project, along with the robots involved, via IEEE Spectrum
Step 1: Inspection
American robots were the first that arrived to map the radioactive hot spots. Later that summer, small tank-like robots out of Chiba Institute of Technology
in Japan arrived. These Quince
robots can crawl over rubble and climb up and down steps on a flexible series of treads, and they’re equipped with cameras and dosimeters to identify hot spots. However, with the plant’s structures interfering with wireless communication, the Quinces have to receive commands through cables, which can get tangled. The team is working on a new bot named Sakura, who automatically picks up the slack in the cable behind it when it changes direction. Sakura also carries a heavy camera that can detect gamma radiation, and it can also plug itself in to recharge (so workers don’t have to handle it once it becomes too radioactive).
Step 2: Decontamination
To cut though obstacles and pick up debris, Hitachi’s ASTACO-SoRa
has two arms that stretch about 8 feet, and it can lift over 300 pounds (though it can’t climb stairs). At the end of each arm, there are grippers, cutting blades, or a drill, depending on the task. Once the path is cleared, the robotic janitors come in. Toshiba
have designed robots that use high-pressure water jets and dry ice to tackle the surfaces of walls and floors. They can also vacuum up the sludge, though their range is defined by their communication cables. Another bot, the Raccoon
, trails long hoses behind it to supply water and suction.
Step 3: Removing spent fuel rods
Hundreds of spent fuel assemblies are contained in a pool of water on the top floor of each reactor. To empty the pools, TEPCO workers will lower a cask into the pool using a crane-like machine and pack the submerged container with fuel assemblies using long mechanical arms. Then the transport cask will be lowered into a truck and brought to a common pool in an intact building.
Step 4: Containing radioactive water
Groundwater flowing through the site ends up mixing with radioactive cooling water leaked from reactor vessels. TEPCO treats that water to remove most of the radioactive elements, and then stores it in massive tanks. The company hopes to stop the flow of groundwater with pumps and underground walls
. And to plug in the holes in reactor vessels, they’re looking into using submersible robots by Hitachi, Mitsubishi, and Toshiba to locate the leaks -- including one that uses ultrasonic sensor to find breaches.
Step 5: Removing the three damaged nuclear cores
There are melted cores inside reactors 1, 2, and 3, and removing them could take at least two decades. First, containment vessels must be flooded with water to shield the radioactive fuel. Submersible robots -- adapted from deep-sea oil well inspectors -- will map the fuel assemblies in the pressure vessels. To break up the mess of metal pooled at the bottom of the pressure vessels, they’ll need long drills capable of reaching 80 feet down (even longer if fuel leaked down into the containment vessel below). Then machines will lift the debris -- which still requires a lot more research to understand -- into radiation-shielded transport casks and take them to a storage facility to be named later.
Image: Fukushima reactors / Japan Ministry of Land, Infrastructure and Transport via Wikimedia
This post was originally published on Smartplanet.com